Environmental concerns and the limited resources of fossil fuels, in conjunction with energy security needs, have increased the importance of renewable technologies (wind and solar) in the electrical energy landscape. See B. Dunn et al., Science 334, 928 (2011). Since renewable energy sources are intermittent, integration with the grid requires reliable energy storage solutions to tailor power generation and supply to demand. See Z. Yang et al., Chem. Rev. 111, 3577 (2011). This would allow for a flexible supply of energy, independent of typical peak consumption profiles. Several energy storage technologies for stationary applications have been developed. See Z. Yang et al., Chem. Rev. 111, 3577 (2011). Electrical energy storage is well equipped to balance the dynamics of demand and supply, however very few technologies can meet the cost-performance targets required for widespread implementation. See C. J. Barnhart and S. M. Benson, Energ. Environ. Sci. 6, 1083 (2013). In addition to cost, safety, reliability via long cycle life, round-trip energy efficiency and minimal maintenance are also important parameters to consider.
Based on these requirements, room temperature Na-ion batteries (NIBs) are becoming increasingly attractive due to: (1) natural abundance and low production cost for Na; (2) reduced safety consideration as compared to traditional rechargeable batteries based on flammable organic electrolytes; and (3) high ionic conductivity, associated with high round-trip efficiency and energy density. See H. Pan et al., Energy Environ. Sci. 6, 2338 (2013); and H. Kim et al., Chem. Rev. 114, 11788 (2014). In this context, there is a need to develop low cost materials with high energy storage capacity and long cycle life for NIBs.
The vast majority of electrodes for NIBs focus on known materials extensively developed for Li-ion batteries. This is primarily due to the fact that Na- and Li-ion systems have related chemistries, albeit distinct kinetic and thermodynamic properties. See S. Y. Hong et al., Energy Environ. Sci. 6, 2067 (2013). Metal oxides and polyanionic type compounds have shown the most promising results for NIB electrodes to date. See H. Pan et al., Energy Environ. Sci. 6, 2338 (2013). Although significant advancements have been achieved in recent years, there is still a need to implement novel materials with tailorable structures and differentiating reaction mechanisms. See X. Xiang et al., Adv. Mater. 27, 5343 (2015); and C. Fang et al., Adv. Energy Mater. 6, 1501727 (2016).
In this context, metal-organic frameworks (MOFs) may be attractive battery electrode candidates owing to their high porosity and tunable framework components. Also, their synthesis typically requires low energy input and relatively inexpensive starting materials. MOFs are three-periodic porous materials constructed from single-metal-ions or metal cluster nodes and organic linkers. See S. R. Batten et al., Pure Appl. Chem. 85, 1715 (2013). Traditional applications of MOFs relate to gas storage and separations, catalysis, and luminescence, to name a few. See B. Li et al., The J. Phys. Chem. Lett. 5, 3468 (2014); M. Eddaoudi et al., Chem. Soc. Rev. 44, 228 (2015); D. F. Sava et al., J. Am. Chem. Soc. 133, 12398 (2011); D. F. Sava Gallis et al., Chem. Mater. 27, 2018 (2015); D. F. Sava Gallis et al., Chem. Mater. 28(10), 3327 (2016); J. Lee et al., Chem. Soc. Rev. 38, 1450 (2009); Y. Cui et al., Chem. Rev. 112, 1126 (2012); D. F. Sava et al., J. Am. Chem. Soc. 134, 3983 (2012); and D. F. Sava Gallis et al., Chem. Mater. 26, 2943 (2014). Over the course of the past few years, the number of studies that focus on electrical conductivity and electrochemistry-related applications is steadily increasing; however, these reports are still scarce. See L. Sun et al., Angew. Chem. Int. Ed. 55, 3566 (2016); S.-L. Li and Q. Xu, Energy Environ. Sci. 6, 1656 (2013); L. Wang et al., Coord. Chem. Rev. 307, Part 2, 361 (2016); A. Morozan and F. Jaouen, Energy Environ. Sci. 5, 9269 (2012); and D. F. Sava Gallis et al., J. Mater. Chem. A 4, 13764 (2016). In particular, there are no studies that focus on the electrochemistry of MOFs as battery electrodes.